David comes to the battlefield finding Goliath taunting the Israeli army. To paraphrase:
Goliath: You are all slaves of Shaul (King Saul.) I have shamed you!
David (to the army): Are you guys going to let this uncircumcised Philistine talk to you like that?
What shame is he referring to? Since a slave can’t have two masters, by accusing them of being slaves of Shaul, he implies they are not slaves of God, a major insult. David brings up the fact that Goliath is uncircumcised, as circumcision is the “branding” of a slave of God, and reminding army of its true master.